IBM 3270 PC/GX Capacitor Replacement Guide

This guide documents capacitor diagnosis and replacement for the IBM 3270 PC/GX (machine type 5371). System-unit cap work is identical to the 3270 PC; PC/GX-specific cap work covers the IBM 5378 Display Attachment Unit (larger and more capable than the /G's 5278) and the IBM 5379 19" Color Display (higher anode voltage and more capacitance than the /G's 14" 5279).

Important Caveat

Per-board exact cap parts lists for the 5371 system unit, the 5378 attachment unit, and the 5379 monitor are not transcribed in any publicly accessible source. This guide documents typical early-1980s practice; verify printed values before ordering replacements.

Safety Warning

The PC/GX has three independent PSUs and a particularly high-voltage CRT to discharge before service:

5371 system unit PSU — standard IBM 130 W switching supply.
5378 Display Attachment Unit — separate mains lead, separate internal linear PSU. Larger than the /G's 5278.
5379 Color Display — separate mains lead. 19" CRT anode voltage is typically 22–25 kV — higher than the /G's 14" 5279, and stores significantly more energy.

Before any work:

Power off and unplug every mains lead.
Wait 30 seconds (longer for the 5379 — large 19" tubes recover residual charge for several minutes).
Discharge each PSU's bulk caps.
Discharge the 5379 CRT anode to chassis ground via a high-voltage probe rated for at least 30 kV. Repeat the discharge after a few minutes' wait.
Verify each PSU and the CRT anode with a multimeter.

System Unit (5371) Capacitors

Identical to the IBM 3270 PC Capacitor Replacement Guide and the 3270 PC/G system unit. The 5371 is the same machine type for both /G and /GX.

RIFA-branded X / Y mains-suppression capacitors in the 130 W IBM PSU — preventive replacement.
Standard XT PSU bulk filter and secondary electrolytics.
Planar through-hole tantalum bypass caps.
Per-card decoupling on Keyboard Adapter, Display Adapter, APA, PSS, 3278/79 Emulation Adapter.

IBM 5378 Display Attachment Unit Capacitors

The 5378 is the rarest single component on a PC/GX after 40 years, and its internal PSU plus higher-resolution vector graphics circuit carry substantial capacitance.

5378 Internal PSU

Larger than the 5278's PSU. Representative cap values:

**5378 PSU capacitor inspection (representative)**
Value	Voltage	Type	Position
4700–10000 µF	16 V	Low-ESR aluminium electrolytic, 105 °C	+5 V bulk × 2 (higher current than 5278)
2200–4700 µF	35 V	Aluminium electrolytic, 105 °C	+12 V bulk
220 µF	35 V	Aluminium electrolytic, 105 °C	−12 V bulk
100 µF	50 V	Aluminium electrolytic, 105 °C	Primary auxiliary × 2
47 µF	50 V	Aluminium electrolytic, 105 °C	Primary startup
470 µF	200 V	Aluminium electrolytic, 105 °C	Primary bulk (lethal-charge)
0.1 µF	275 VAC	X2 class	Mains suppression — replace if RIFA-branded

5378 Vector Graphics Board

The 5378's vector graphics board is larger than the 5278's because of the higher resolution and expanded frame buffer. Representative caps:

**5378 vector graphics board capacitor inspection (representative)**
Value	Voltage	Type	Position
10 µF	16 V	Tantalum bypass	IC bypass throughout (more ICs than the 5278)
22 µF	16 V	Tantalum bypass	Frame buffer RAM bypass × multiple
47 µF	16 V	Aluminium electrolytic	Local +5 V power filter
100 µF	16 V	Aluminium electrolytic	Output drive stage filter (75-pin output)
220 µF	16 V	Aluminium electrolytic	High-current output stage filter

IBM 5379 19" Color Display Capacitors

The 5379 has a larger internal PSU and a more powerful deflection board than the 5279, reflecting the 19" tube's higher anode voltage and larger deflection requirements:

5379 Internal PSU

**5379 19" CRT PSU capacitor inspection (representative)**
Value	Voltage	Type	Position
4700 µF	25 V	Low-ESR aluminium electrolytic, 105 °C	+12 V bulk smoothing
2200 µF	16 V	Low-ESR aluminium electrolytic, 105 °C	+5 V bulk smoothing
470 µF	35 V	Aluminium electrolytic, 105 °C	+12 V auxiliary
220 µF	200 V	Aluminium electrolytic, 105 °C	Primary bulk (lethal-charge)
0.1 µF	275 VAC	X2 class	Mains suppression — replace if RIFA-branded

5379 Deflection / Flyback Board

**5379 deflection / flyback board capacitor inspection (representative)**
Value	Voltage	Type	Position
1–10 µF	25 V	Aluminium electrolytic	Vertical / horizontal oscillator bypass
22–100 µF	35 V	Aluminium electrolytic	Vertical deflection driver (larger than 5279 because 19" tube needs more deflection current)
220–680 µF	35 V	Aluminium electrolytic	Deflection rail bulk × 2
0.01–0.1 µF	2 kV	Ceramic disc (HV)	Snubber on flyback collector
0.0027–0.01 µF	2 kV	Polypropylene HV	Horizontal deflection tuning

The 19" 5379 phosphor is particularly susceptible to burn-in from sustained CAD / GDDM use — a burned-in tube cannot be field-repaired, only replaced with a donor.

Recommended Modern Replacements

Same guidance as the IBM 3270 PC Capacitor Replacement Guide and IBM 3270 PC/G Capacitor Replacement Guide: Panasonic FR/FM/FC, Nichicon HE/HZ (post-2007 date codes), Rubycon ZLH/ZLJ/YXJ, United Chemi-Con KZH/KZE; 105 °C; equal capacitance, equal-or-higher voltage; verify lead spacing.

Recap Procedure

Discharge all three PSUs (5371, 5378, 5379) plus the 5379 CRT anode.
Service each subsystem separately on different bench days if possible — the PC/GX has more capacitor count than any other PC family system on the wiki.
Photograph each board from both sides at high resolution.
Mark each cap's polarity before desoldering.
Desolder with solder wick at no more than 350 °C, 5–7 seconds per cycle.
Fit replacements matching silkscreen polarity.
Solder both leads; inspect for clean fillets; trim flush.

Post-Recap Verification

5371 system unit — verify all rails on the bench under a 2 A resistive load.
5378 attachment unit — verify all rails; verify the 75-pin output is responsive.
5379 19" monitor — verify all rails; verify HV at the anode (with HV probe); verify a clean raster appears; check geometry on a known-good test pattern.
Reassemble; boot GCP; verify mainframe graphics render at 1024 × 1024 on the 5379.

When Not to Recap

If the PC/GX chassis POSTs cleanly, the 5378 produces valid 1024 × 1024 output, the 5379 displays without geometry distortion or visible burn-in, GCP loads, mainframe GDDM graphics render cleanly, and the system runs reliably, the caps are within tolerance.

Always recap if:

RIFA-branded X / Y mains-suppression cap present in any of the three PSUs (preventive replacement).
Visible cap failure on the 5371 planar, any card, the 5378, the 5379, or any of the three PSUs.
PSU smoke, fishy odour or audible whine from any unit.
5378 output garbled or absent at 1024 × 1024 — most commonly the 5378's high-current PSU caps.
5379 geometry distortion or shifted raster — deflection-board electrolytic ageing.
System unstable when warm but stable when cold.

Related Pages

IBM 3270 PC/GX
IBM 3270 PC/GX Maintenance Guide
IBM 3270 PC/GX Troubleshooting Guide
IBM 3270 PC/G Capacitor Replacement Guide — smaller-display sibling
IBM 3270 PC Capacitor Replacement Guide — shared system-unit cap work
Capacitor Failure Symptoms

References